The present invention relates to non-invasive pulse oximetry. More specifically, the present invention relates to a method for detecting the venous blood component in a pulse oximetry signal.
Non-invasive photoelectric pulse oximetry for determining blood flow characteristics is well known in the art. Illustrative are the methods and apparatus described in U.S. Pat. Nos. RE 35,122; 5,193,543; 5,448,991; 4,407,290; and 3,704,706.
Pulse oximeters typically measure and display various blood constituents and blood flow characteristics including, but not limited, to blood oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the flesh and the rate of blood pulsations corresponding to each heartbeat of the patient. The oximeters pass light through human or animal body tissue where blood perfuses the tissue such as a finger, an ear, the nasal septum or the scalp, and photoelectrically sense the absorption of light in the tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
Two lights having discrete frequencies in the range of xcx9c650-670 nanometers in the red range and xcx9c800-1000 nanometers in the infrared range are typically passed through the tissue. The light is absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light passed through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption.
The output signal from the pulse oximeter, which is sensitive to the arterial blood flow, contains a component that is waveform representative of the patient""s blood gas saturation. This component is referred to as a xe2x80x9cplethysmographic wave or waveformxe2x80x9d (see curve P in FIG. 1).
A problem generally associated with non-invasive pulse oximeters is that the plethysmograph signal (and the optically derived pulse rate) may be subject to irregular variants in the blood flow including, but not limited to, motion artifacts, that interfere with the detection of the blood constituents. A motion artifact is caused by the patient""s muscle movement proximate to the oximeter sensor, for example, the patient""s finger, ear or other body part to which the oximeter sensor is attached, and may cause spurious pulses that are similar to pulses caused by arterial blood flow. These spurious pulses, in turn, may cause the oximeter to process the artifact waveform and provide erroneous data. This problem is particularly significant with infants, fetuses, or patients that do not remain still during monitoring.
A further problem is that the plethysmograph signal includes the blood oxygen saturation signals of the venous (i.e., non-pulsating) and arterial (i.e., pulsating) blood. The inability to reliably detect the venous blood component in the optical signal could, and in many instances will, result in erroneous data.
Several signal processing methods (and apparatus) have been employed to reduce the effects of the motion artifact(s) on the measured signal(s) and, hence, derived plethysmograph waveform. Illustrative are the methods and apparatus disclosed in U.S. Pat. Nos. 4,934,372 and 5,490,505.
In Co-pending application Ser. No. 09/815,827, filed Mar. 23, 2001, entitled xe2x80x9cMethod and Apparatus For Determining Physiological Characteristicsxe2x80x9d, a unique method and apparatus is disclosed that employs an xe2x80x9cindividualizedxe2x80x9d, substantially noise free plethysmographic waveform as a reference signal. The noted reference is transmitted to a correlation canceler that provides a derived plethysmographic waveform that is substantially representative of the subject""s true plethysmographic waveform.
Several methods and apparatus have also been employed to detect the venous blood component in an optical signal. One method relies upon the quantitative measurement in the change in absorbance at each wavelength, as in U.S. Pat. No. 4,407,290 and European patent Nos. EP 0 104 771 A3 and EP 0 102 816 A3. It is also well known that the derivative of the change in absorbance and a peak to peak measurement of the pulsating absorbance component may be used to calculate the oxygen content of arterial blood, as disclosed in U.S. Pat. Nos. 4,407,290 and 4,167,331.
It is further known that a single light detector may be employed. However, when a single light detector is used, the detected light for each wavelength must be separated. This is accomplished by using time separation and synchronous detection, as disclosed in U.S. Pat. Nos. 4,407,290; 4,266,554; and 3,647,299, for example. However, since the light detectors also detect ambient light, some type of ambient light rejection technique is normally employed. One technique is to use four clock states and to allow for the subtraction of ambient light, as disclosed in U.S. Pat. Nos. 4,407,290 and 4,266,544. Another technique is to remove the non-pulsating absorbance component, since ambient light is usually a non-pulsating absorbance frequency, as disclosed in U.S. Pat. Nos. 4,167,331 and 3,998,550.
There are several drawbacks associated with the noted technologies and apparatus. Among the drawbacks are the cost and complexity of the disclosed apparatus.
It is therefore an object of the present invention to provide a cost effective, reliable means of determining the venous blood component in an optical pulse oximeter signal.
In accordance with the above objects and those that will be mentioned and will become apparent below, the method for determining the blood constituents of a patient in accordance with this invention comprises coupling an oximeter sensor arrangement to a tissue region of the patient; passing first and second lights through the patient""s tissue region for a first period of time while the venous blood in the tissue region has a first volume and for a second period of time while the venous blood in the tissue region has a second volume, the first light being substantially in a red light range and the second light being substantially in an infrared light range; detecting a red light signal and an infrared light signal, the red and infrared signals having at least first and second frequencies; computing a first ratio of the red and infrared signals at the first frequency; computing a second ratio of the red and infrared signals at the second frequency; comparing the first and second ratios to determine a first blood constituent.